Sensor unit, process and device for inspecting the surface of an object

ABSTRACT

A sensor unit ( 50 ), a device, and a process for inspection of a surface ( 10′, 10 Δ) of an object ( 10 ) for the purpose of identifying surface characteristics, such as structural defects. The device contains an emitting module ( 51 ) and a receiving module ( 52 ). The emitting module emits at least one beam bundle ( 6, 6′, 6 ″). The receiving module has at least one light receiver ( 15, 16, 20 ). A rotating polygonal mirror wheel ( 2 ) is located in the focal point of a parabolic mirror ( 1 ). A beam bundle ( 6 ) of the laser ( 3, 4 ) is directed onto the mirror ( 1 ) by means of a telecentric lens, which guides the emitter and receiver beam on the same optical axis, whereby the parabolic mirror ( 1 ) guides the deflected beam bundle ( 6, 6′, 6 ″) under a constant angle relative to the axis of symmetry ( 7 ) of the parabolic mirror ( 1 ) along a scanning line ( 23, 24 ) over the object ( 10 ). The diffusely reflected beam bundle, after being deflected out of the common beam path, impinges on the processing unit ( 5 ).

BACKGROUND OF THE INVENTION

The invention concerns a device and a process for the inspection of thesurface of an object.

With respect to the surface inspection of materials, it is known to scanthe respective surface with charge-coupled device (CCD), line, or matrixcameras as well as with laser scanners. It is also known in the art toanalyze the gray-scale value or color pictures with image processingmeans.

In the case of the processing of wood, for example in door and windowmanufacture, or in the fabrication of veneer sheets, it is necessary toinvestigate and determine the quality of the wood to be processed. Indoing so, it should be determined whether the wood has shakes, fissures,knot holes, protrusions or indentations. It should also be determinedwhether the wood has blue stain or red ring rot, which makes themunsuitable for the foreseen purpose.

Prior to the present invention, wood surfaces have normally beenmanually inspected. It is up to now practically impossible toautomatically identify wood which is affected by blue stain or red ringrot. There are a number of technical problems, which generally areassociated with the great depth of focus and the simultaneously highresolution called for by the process as well as with the transportationspeed of the wood. For this, relatively elaborate illumination equipmentwith a very high performance is necessary.

When illuminating wood by means of a laser beam, the so-called scattereffect occurs, which means that a part of the light is dispersed intothe wood fibers. The light scatters in the vicinity of the surface in afunction of the local density distribution. In the case of anundisturbed fiber orientation, a characteristic dipole distribution inthe spatial intensity distribution of the diffusely reflected light ismanifest, whereby the (1/e) drop, the integral intensity, as well as theactual structure of the maxima of the emissions are dependent on thetype of material and on the structure of the defect. ThroughSE-A-7500465-5, a process and a device utilizing a helium-neon laser hasbecome known, where the scatter effect is indirectly exploited for theevaluation.

Through EP-0 198 037 B1, a process for measuring the fiber angles in afibrous material, such as wood, has become known. In the EP '037process, an area on the surface of the material is illuminated with animpinging ray of light and photo-sensitive devices are spatiallyarranged in such that they measure the light reflected by theilluminated area. The fiber angle is measured relative to threereference axes perpendicular to one another (x, y, z) and any point onthe surface of the material is defined as the point of origin of theaxes. The illuminated area encompasses the point of origin and has adiameter, which is at least ten times the size of the average fiberdiameter of the substance to be measured. A majority of thephoto-sensitive devices are positioned so as to be able to assess theazimuthal angular positions around the point of origin of the intensitymaximum of the reflected light. Furthermore, a number of arbitrarypoints in transverse and longitudinal direction of an area on thesurface of the material are staked out, in order to be in a position toassess the azimuthal angular positions of the intensity maxima at eachof the points. By means of the relationship between the azimuthalangular position of the reflected light maxima and the fiber angle, forevery measuring point the corresponding fiber angle is calculatedrelative to all three axes in order to indicate the complete pattern ofthe fiber angles within the measured area of the fibrous material. Forcarrying out this process, a highly elaborate installation is necessaryin order to, on the one hand, measure the radiated beam proportion ofthe reflected light and, on the other hand, measure the proportion ofthe diffusely reflected light.

DE-A-196 04 076.0 proposes a device for the inspection of the surface ofwood for the purpose of determining surface characteristics. The DE '076device includes an opto-electronic sensor, an electronic and/or opticalprocessing unit, a computer capable of real-time operation. With the De'076 device, the wood can be moved relative to the sensor, as well as anincremental position transducer, which synchronizes the sensor with thespeed of the wood. The sensor consists of a color laser scanner with atleast two beam bundles of differing wave lengths and a receiver with twochannels with one opto-electrical receiving element each. The channelsare formed by beam splitting of the reflected beam bundle. A lens forcreating an intermediate image plane is located in at least one of thechannels. After the lens, within one of the channels there is an opticalgraduated filter, which is capable of modulating the passing lightcurrent to the opto-electrical receiving element belonging to itindependent of the position. The signals of the receiving element of thechannel without the graduated filter are converted into a color image inthe computer. The signals of the other channel, the light current ofwhich has been modulated independent of the position, are converted intoa profile image of the surface.

It is, for example, known from U.S. Pat. No. 4,286,880 and JP 59 040 149A that wood surfaces can be investigated by scanning with a light beam.These two documents divulge a rotating mirror, which is located in thefocal point of a parabolic mirror. Light is emitted onto the rotatingmirror from a light source and from there distributed by reflection onthe parabolic mirror along a scanning line on the wood surface. In U.S.Pat. No. 4,286,880, the subject is an improvement of work stations forthe localization of wood defects, wherein an operating person has tofind the defects. The operating person marks the defects and the marksare detected by an optical sensor with a binary output signal. Theobject of JP 59 040 149 A is the detection of live knots by means ofasymmetrically scattered light. For the solution of this problem, a woodsurface is scanned by a light beam and light scattered from the woodsurface is directly detected by two detectors arranged symmetricallywith respect to the scanning beam.

From the state of the technology, optical distance sensors are alsoknown. The distance sensor divulged in EP-0 387 521 A2 is based on atriangulation process. A beam of light is focused on the surface to bemeasured by means of a lens. Light scattered by the surface is collectedby the same lens and focused on position-sensitive detectors by aconcave mirror. The components are positioned relative to an opticalaxis such that a high light sensitivity is assured by a small angle ofincidence. Another distance sensor, divulged in WO 93/11403, contains arotating polygonal mirror, which distributes light emitted from a lightsource onto a scanning line on the surface to be measured. A scanninglens projects the point-shaped light source onto the surface. The lightreflected by the surface is projected onto a point-shaped detector bymeans of the same scanning lens and a further lens. A maximum lightintensity impinges on the detector only when the object is in the focalplane of the scanning lens and when the detector is simultaneously inthe focal plane of the further lens. The detected light intensity istherefore a measure for the distance of the object from the scanninglens. The construction can be refined by utilizing several detectors,each of which supply maximum signals at differing object distances.

SUMMARY OF THE INVENTION

The present invention is directed toward a device and a process for thedynamic inspection of the surfaces of objects such as wood, tile,textile, and glass. The present invention is further directed toward adevice and process for the identification of surface characteristics.With the present invention an automatic inspection of the surface can becarried out continuously and with a high speed. The continuous, highspeed inspection provided by the present invention permitscharacteristics such as shakes, fissures, cracks, knot holes,protrusions, indentations, and, in the case of wood, blue stain or redring rot, to be identified with certainty by the exploitation of thescatter effect. In particular, simultaneously the position-dependentdiffuse reflection of a surface, the distance of the surface as well asthe deviation of the diffuse reflection characteristic as a function ofthe position can be detected by a Lambert projector at processing speedsof several meters per second in real time. Apart from this, the deviceshall be of a simple construction and should be able to be manufacturedat a low cost.

In accordance with the present invention, a device for inspecting asurface of an object for the purpose of identifying surfacecharacteristics contains a sensor unit and a lens. The device also has ascanning device, to which light is transmitted from the sensor unit. Thescanning device comprises a concave mirror, in the focal point of whichis located a light deflecting element that is illuminated by the sensorunit and having with a deflection angle dependent on time. Accordingly,the light transmitted by the sensor unit can be guided over the objectalong a scanning line and light from the scanning device diffuselyreflected by the object impinges on the sensor unit.

In further accordance with the present invention, the device includes aparabolic mirror, in the focal point of which a rotating polygonalmirror is located, onto which the beam bundle of the laser is directedby means of a telecentric lens, which guides the transmitter andreceiver beam along the same optical axis. The parabolic mirror guidesthe deflected beam bundle over the object along a scanning line under aconstant angle relative to the symmetry axis of the parabolic mirror,and guides the beam bundle diffusely reflected by the object back alongthe same path. The beam bundle, after being reflected out of the commonbeam path, impinges on the processing unit.

The device is based on the fact of the so-called Tracheid effect(scatter effect) which occurs with density changes on the surface in thecase of a number of materials with a point-shaped, coherentillumination. In doing so, light enters into the material through thesurface and is guided inside the material. The guidance of the lightwithin the material and its damping are determined by the structure ofthe material. The device and the process exploit the scatter effect forthe evaluation of surface anomalies and carry out an assessment ofsurface defects under real time conditions.

In the beam path between the laser and the polygonal mirror wheel thereis a mirror with an aperture or hole, through which the irradiated beambundle passes and impinges on the polygonal mirror wheel. The mirrordeflects the diffusely reflected beam bundle onto a lens at a givenangle. Optical detectors and the electronic processing unit, inpreference including a computer capable of real time operation, aresituated in the focal plane of the lens. After the lens and in front ofits focal plane, two beam bundles and preferably at least two channelsare created by beam splitting, which are evaluated separately. A highprecision of execution is important for the functioning of the device.

If, for example, the material to be inspected is wood, then around theintensive light spot of the direct irradiation two light cones areformed, aligned to the direction of the fibers. The light cones diminishcorresponding to the dipole characteristic and join the direct lightspot of the impinging beam. In correspondence with the change of thesurface structure and the direction of the fibers, the two light coneschange with respect to their length, brightness and direction. Thebrightness, length and direction are dependent on the local defect andits shape as well as on the direction of the fibers. The inventors havediscovered that, through the scatter effect, for the first time in thecase of wood just starting blue stain or red ring rot can be madevisible, long before the attack can be identified by the unaided eye,chemical analyses or microscopic viewing. Equally on the basis of thescatter effect defects in the wood, such as, for example, knot holes,compression, red or blue stain become visible, which lead to adiminution of the quality of the corresponding wood.

In order to obtain a surface profile (3D - profile) by means of atriangulation process, at least one light beam emitted from the sensorunit can be guided onto the object. Light diffusely reflected from theobject under a finite angle relative to the impinging beam can be guidedback to the sensor unit in such a manner that the impinging and thereturned light beam are essentially in coincidence in a plane parallelto the surface of the object. In an alternative version, plural laserbeams can be guidable by means of various mirrors that are offsetrelative to one another such that the individual laser beams are incoincidence in the horizontal plane, while in the vertical object planea constant angle is given, in order to measure the surface profile bymeans of the vertical deposition of the diffusely reflected laser lightwith a position-sensitive opto-electrical receiving element, which is inparticular a PSD sensor element capable of high speed.

In further accordance with the present invention, at least one laserbeam is focused on the surface of the object in order to obtain a 3Dsurface profile by triangulation. The vertical deposit of the diffuselyreflected laser light is measured through an additional guidable mirrorunder a constant angle by means of a position-sensitive receivingelement (PSD) capable of high speed.

A process for the inspection of the surface of an object to identifysurface characteristics in accordance with the present inventionutilizes a sensor unit that emits light to a scanning device, andwherein the object is moved relative to the sensor unit. The emittedlight is focused on the object by means of a telecentric projection andguided over the object along a scanning line under a constant anglerelative to the object to be scanned, and relative to the vertical lineof the transportation surface. Light diffusely reflected by the objectis guided back to the sensor unit along the same beam path as theemitted light.

In accordance with another embodiment of the inventive process, thelaser beam is focused on the object by means of a telecentricprojection, which guides the emitted and received beam along the sameoptical axis, focuses it on the object, and guides it along a scanningline always under a constant angle relative to the object to be scanned,and relative to the vertical line of the transportation surface of theobject. This can be the most effectively achieved by the arrangement ofa parabolic mirror, in the focal point of which a polygonal mirror wheelis located, so that the angle of the laser beam is constant with respectto the symmetry axis of the parabolic mirror. The spatial resolution islimited solely by the focusing ability of the laser light. Thetelecentric beam bundle of the moving laser light spot, the measuringpoint, along a scanning line, does not have to impinge orthogonally onthe surface to be scanned, the angle of impingement can rather be anyone within wide ranges. The angle must, however, be constant relative tothe normal line of the transportation surface of the object. In doingso, the diffusely reflected light is guided back to the detector throughthe same projection system.

In further accordance with the process, various surface characteristicscan be measured in real time, namely:

a) the intensity distribution of the diffusely reflected laser light,and/or

b) the distribution of the intensity of the laser light scattered bylocal density variations (Tracheid effect), which is observed throughspatial filters in the scatter channel, and/or

c) the elevation profile (3D channel) of the surface, which is measuredby means of a triangulation process, and/or

d) double refraction characteristics, which are measured by means ofdetection processes dependent on polarization, for example by means ofan analyzer parallel and anti-parallel to the surface direction.

After the lens, and in front of the image plane of the lens, at leasttwo channels are formed by beam splitting into two partial beam bundles,which are assessed. On principle, it is even possible to detect allcharacteristics mentioned above with only one laser, which can beimplemented by a modification of the receiving module. For splitting thediffusely reflected laser radiation into partial radiation bundlesaccording to their differing wave lengths, a dichroic mirror ispositioned in the beam path. From the partial beam bundles of differingwave lengths various surface characteristics, such as the elevationprofile in the 3D channel, the reflectivity in a red light channel aswell as the Tracheid effect in the mentioned scatter channel aresimultaneously recorded with a high repetition rate. One of the partialradiation bundles formed by the dichroic mirror, preferably the redlight proportion of the diffusely reflected laser beams, is again splitinto two channels by means of a semi-transparent mirror, in which inaccordance with the diffusely reflected laser light sensitive sensorsare located. Within one channel the image of the directly diffuselyreflected light point or spot on the object is evaluated and the imageof the light cones of the scatter effect is blanked out and from theimage of the laser point or spot a gray-scale image is obtained. Insidethe other channel (scatter channel) by means of special spatial filtersthe directly diffusely reflected light point or spot is blanked out andonly the image of the remaining light cones is evaluated. Blanking outis accomplished by means of special spatial filters.

The Tracheid—scattered laser light (scatter channel), which, forexample, in the case of wood serves for the evaluation of the image ofthe remaining light cones, is detected in a real time process for makingvisible density-dependent surface anomalies, such as shakes, cracks,fissures, structural defects. The density-dependent surface anomaliesare detected by means of a four-quadrant process, for example by meansof dichroic mirrors or a four-quadrant diode, position dependent in theform (S_(x)+S_(y))/S and in the direction arctan (S_(x)/S_(y)), ifnecessary in combination with the triangulation process in the 3Dchannel also in function of the elevation, the spatial resolution ofwhich is only limited by the focusing ability of the laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further features of the present invention will be apparentwith reference to the following detailed description and drawings,wherein:

FIG. 1 is a schematic cross section of a first embodiment of the sensorunit in accordance with the invention;

FIG. 2 is a schematic cross section of a second embodiment of the sensorunit in accordance with the invention;

FIG. 3 is a schematic cross section of a third embodiment of the sensorunit in accordance with the invention;

FIG. 4 is a top view of a deflecting element of the sensor unit of FIG.3;

FIG. 5 is a schematic layout of the device in accordance with theinvention in top view;

FIG. 6 is a top view of a technical embodiment of the device inaccordance with the invention, in which the beam path is folded in orderto achieve a small depth of the construction;

FIG. 7 is a side view of the device of FIG. 6;

FIG. 8 is a view of a receiving module with a spatial filter vertical tothe parabolic mirror in the scatter channel; and,

FIG. 9 is a view of the receiving module of FIG. 8 with the same spatialfilter parallel to the parabolic mirror in the scatter channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic cross section through the first embodiment ofthe sensor unit 50 in accordance with the invention. The sensor unit 50serves to emit light to a scanning device 60 (FIGS. 5-6, not illustratedin FIG. 1), which is situated in the direction of an arrow 61, and forreceiving light impinging from the scanning device 60. The sensor unit50 contains a light-emitting sender module 51 and a light receivingreceiver module 52. The emitting module 51 contains at least one lightsource 3, which can be a laser, a light-emitting diode (LED) or anotherlight source. In front of the light source 3, if necessary, there can bea focusing optical system (not illustrated). The receiving module 52contains at least one light receiver 20, such as a photo-diode, a CCDcamera, a position-sensitive receiving element (PSD), etc. An opticalsystem 13, such as a focusing lens or a multi-element lens, can projectan object (not illustrated in FIG. 1) onto the light receiver 20.

The sensor unit 50 also has an optical deflection element 9. If lightfrom the direction of the scanning device 60 impinges on the sensor unit50, the light is split-up by the deflecting element 9 into first andsecond light beam paths 53, 54, which differ from one another. The firstbeam path 53 is defined by a first spatially limited part 55 of thelight. The second beam path 54 is defined by a second spatially limitedpart of the light 56. The first beam path 53 has a smaller crosssectional area than the second beam path 54. The emitting module 51 islocated in the first beam path 53 and the receiving module 52 is locatedin the second beam path 54. In this embodiment the deflection element 9is designed as a plane mirror with an aperture 25 and arranged such thatlight traveling to the deflection element 9 is, to a greater extent,left to pass through to the scanning device 60 through the aperture 25.Light emanating from the scanning device 60 outside the first beam path53 is, in contrast, to a greater extent reflected to the receivingmodule 52. The surface shell, which surrounds the aperture 25 in thedeflection element 9, is in preference parallel to the direction ofdiffusion of the emitted light 53.

In FIG. 2, a second preferred embodiment of the sensor unit inaccordance with the invention 50 is schematically illustrated. Here thedeflection element 9 is designed as a small mirror and arranged suchthat light 53 traveling from the emitter module 51 to the deflectionelement 9 is, to a greater extent, reflected to the scanning device 60(not illustrated in FIG. 2). Light 54 emanating from the scanning deviceoutside the first beam path 53 is, in contrast, to a greater extentallowed to pass through to the receiving module 52.

FIG. 3 schematically illustrates a further preferred embodiment of thesensor unit in accordance with the invention 50. The deflection element9 corresponds to that of FIG. 1. This embodiment contains two lightsources 3, 4, for example, two lasers. A first laser 3 emits red lightin the wave length range between about 620 nm and 770 nm, preferably 680nm. A second laser 4 emits infrared light in the wave length range above770 nm, preferably 830 nm. The two laser beams are joined by means of amirror 12 and a beam splitter 11.

The receiving module 52 contains two light receivers 15, 20 and a beamsplitter 14. Light 54 impinging on the receiving module 52 is split-upby the beam splitter 14 and delivered to the light receivers 15, 20. Thebeam splitter 14 can have dichroic characteristics. In other words, thereflection to transmission ratio of the splitter 14 can be dependent onthe wave length.

FIG. 4 schematically illustrates a top view of the mirror 9 withaperture 25 of FIG. 3 along the line IV—IV. Light 54 emanating from thescanning device 60, which exactly coincides with the light 53 travelingto the scanning device 60, does not impinge on the receiving module 52.Rather, the portion of light emanating from the scanning device 60 thatcoincides with the light 53 traveling to the scanning device 60 willpass through the aperture 25. This way a “cross-talk” is efficientlyprevented.

In accordance with FIG. 5, the fundamental principle of the device inaccordance with the invention consists of a concave mirror 1, which ispreferably cut out of a paraboloid as a narrow strip having length 1 andheight h. Preferred as a concave mirror is a parabolic mirror, becauseit provides an ideal, practically aberration-free image. Within thefocal point of the parabolic mirror 1 is a polygonal mirror wheel 2,which is rotated by a motor at a high speed, with one of its polygonsurfaces arranged in such a manner, that preferably at an angle of 45degrees to the normal line of the polygon surface of the mirror wheel tothe optical axis 7 (symmetry axis) of the parabolic mirror 1, the centerof the polygon surface comes to lie exactly in the focal point of theparabolic mirror 1.

Two lasers 3, 4 each generate a laser beam. One laser preferablyoperates in the wave length range of approximately 680 nm, therefore inthe red light range. The other laser preferably operates in the wavelength range of 830 nm, therefore in the infrared light range. The laserbeams are deflected by mirrors 11, 12 and brought together into a commonbeam 6. For this purpose, the mirror 11 illustrated in FIG. 1 istransparent for the laser beam of the laser 3 situated behind it.

The combined laser beams 6 pass through a hole 25 in a further mirror 9and impinge on one of the plane polygon surfaces of the rotatingpolygonal mirror wheel 2. Dependent on the design of the polygonalmirror wheel 2 and on the center distance of it from the parabolicmirror 1, the polygonal mirror wheel 2 guides the laser beams 6, 6′, 6″at a certain predefined horizontal image field angle α over theparabolic mirror 1 in its longitudinal expanse 1, as can be seen in FIG.5.

The horizontal image field angle a is limited by the laser beams 6′, 6″.The laser beam 6′″ is reflected by the parabolic mirror 1 and guidedparallel to itself over the longitudinal expanse of the parabolic mirror1 and forms the scanning line. The reflected laser beam 6′″ is guided toan inclined mirror and impinges on the surface 10′ of an object to bescanned, for example a piece of wood traveling with the speed v relativeto the laser. In this manner, the laser beams 6 from the lasers 3, 4,which are focused on the object 10, are guided under a constant anglerelative to the symmetry axis 7 of the parabolic mirror 1, i.e., to thenormal line of the transportation surface of the object 10, to theobject 10 to be scanned, along the scanning line 23, 24 (i.e., FIG. 7).The object surface 10′ is projected onto the light receiver contained inthe receiving module as a telecentric image.

As discussed hereinbefore with respect to FIGS. 1-4, the laser lightdiffusely reflected in the point of impingement is guided back along thesame path, so that the arriving beam and the diffusely reflected beam inessence coincide. The polygonal mirror wheel 2 projects the diffuselyreflected beam bundle onto the mirror 9, which deflects the reflectedbeam bundle and guides it to a processing unit 5. The processing unit 5,which evaluates the diffusely reflected beam bundle optically andelectrically, comprises a computer capable of real time operation.

In the FIGS. 6 and 7, a technical embodiment of the device isillustrated. The two lasers 3, 4 generate laser beams 6, which arebrought together by mirrors 11, 12. The laser beams 6 are projected ontothe rotating polygonal mirror wheel driven by a motor 27, rotating athigh speed, through a hole 25 within the mirror 9. The mirror 9 islocated in the focal point range—this time relative to a longish, planemirror 17—of the parabolic mirror 1. The two laser beams 6 are projectedonto the mirror 17 by the polygonal mirror wheel 2, which reflects thelaser beams onto the parabolic mirror 1, so that the folded beam pathillustrated in FIGS. 6 and 7 is produced.

The parabolic mirror 1 now has the effect that the laser beams reflectedby it can be guided parallel to one another onto the surface 10′. Thereflected beams, therefore, are at a constant angle relative to itssymmetry axis 7, relative to a normal line 29 of the transportationsurface 10′ of the object 10.

To do this, in the beam path after the parabolic mirror 1, as shown inFIG. 7, there are two plane mirrors 18, 19. The plane mirrors 18, 19 arealigned transverse to the surface 10′ of the object 10 to be scanned andguide the laser beams traveling parallel in one another along a scanningline 23 over the surface 10′. Two mirrors 18, 19 are used in order to,with respect to the evaluation in the triangulation process, obtain a 3Ddepiction of the image. If the information in the direction of thevertical axis is not required, then only one mirror 19 is necessary forthe construction of the device and for carrying out the process.

In the following, with reference to FIG. 7, beam paths for the casewithout triangulation process and for the case with triangulationprocess are discussed. The light beam emitted by the sensor unit 50 andreflected by the parabolic mirror 1 impinges on the object 10 throughmirror 19. The angle of impingement β of the light 10′ relative to thenormal line 29 is greater than zero. In other words, the light is notperpendicular to the surface 10′. As a consequence, light directlyreflected from the surface 10′ does not travel back to the mirror 19,but rather travels away from the mirror. Therefore, diffuse reflectionon the surface 10′ is measured.

In the case without triangulation process, light is measured, whichreturns to the sensor unit 50 under the same angle of reflection βthrough the mirror 19 on the same path as the impinging light. If atriangulation process is to be utilized, then the vertical deposition oflight is measured, which is diffusely reflected under a constant angle θrelative to the impinging light and returns to the sensor unit 50through a further mirror 18. Also in this case, the light in essencetravels along the same path back to the sensor unit 50 as the impinginglight. In this case, the angle θ determines the resolution of the 3Dmeasurement. The greater the angle θ, the more sensitively the 3Dprofile can be measured. A preferred value for the angle θ isθ=15.50°±1.5°. In an alternative version of the process, light can bebeamed onto the object 19 both through mirror 19 as well as throughmirror 18—or even on more than two light paths, and the respectivediffusely reflected light portions detected.

Two further mirrors 21, 22, which in the top view are arranged laterallyfrom the mirrors 18, 19 and, if necessary, in different planes, serve tosimultaneously guide the laser beams over a side surface 10″ of theobject 10 and along a further scanning line 24. Accordingly, a furtherimage is obtained, also as a 3D image, so that simultaneously two planes10′, 10″ inclined relative to one another at a given angle can bescanned. In the illustrated example the surfaces are inclined 90 degreesto one another. The diffusely reflected light beams travel back alongthe same path (i.e.—via parabolic mirror 1 and mirror wheel 2) andimpinge on the mirror 9 under the horizontal image field angle α, fromwhere they are deflected towards a lens 13.

With respect to FIG. 6, a dichroic mirror 14 is located in the beam pathof the lens 13. The dichroic mirror 14 is transparent to infraredradiation of the diffusely reflected laser light, but deflects thediffusely reflected red light radiation of the other laser. After themirror 14, in the image field plane of the lens 13 there is a receiver20, the received signals of which are utilized as 3D information. Withthis information, a relief image can be calculated in the computer,which enables the identification of depth changes of the object to beinspected. A position-sensitive, opto-electrical receiving element, inparticular a PSD sensor element capable of high speed, is preferablyused as a sensor element 20 for the 3D channel. The PSD sensor elementdetects the positional deviation of the laser beam relative to the zeroposition, which has been guided through the mirror 18, 21.

The red light portion of the diffusely reflected laser beams isdeflected through the dichroic mirror 14 and impinges onto a splittermirror 26. The splitter mirror 26 splits-up the red light portion intotwo channels in which light-sensitive sensors 15, 16 are located. Onechannel is operated as a so-called direct red sensor and provides agray-scale image, wherein the image of the direct light point or spot onthe object is evaluated. For this purpose, by means of a diaphragm thelight cones of the scatter effect are blanked out. The other channel isthe so-called scatter channel and serves to evaluate the actual scatterimage and, thus, in the case of wood, the light cones, which adjoin thedirect light spot. To do this, the center point or center spot, whichis, of course, evaluated in the other channel, is blanked out be meansof special spatial filters 30 in the scatter channel (FIGS. 8-9). Theimage of the remaining light cones, for example, is projected on to afour-quadrant diode. The position of the cones relative to one anotherand relative to the direction of transport can be calculated from therelationship of the two light cones to one another. Therefore, in thecase of wood, for example, the fiber direction or places affected withblue stain or red ring rot can be identified. The evaluation of thediffusely reflected laser radiation is, therefore, carried out such thatthe energetic and/or the positional distribution of the diffuselyreflected radiation is converted into different electrical signals.

By means of a computer capable of real time operation, the channels canbe subsequently evaluated and the images generated displayed on amonitor. It is equally possible to convert the signals from the threechannels into color values, in order to thus also generate a colorimage.

It is also conceivable to transmit the diffusely reflected laserradiation to a CCD camera for evaluation. FIGS. 8 and 9, on one hand,show a view of the receiving module 15 with a spatial filter 30 verticalto the parabolic mirror 1 in the scatter channel with lens 13 and mirror14. On the other hand, FIGS. 8 and 9 show a view of the same receivingmodule 15 with the same spatial filter 30 parallel to the parabolicmirror in the scatter channel. One can make out the spatial blanking-outof the center spot by a central plate 31, whereby the light conesimpinge on the light receiver 15 through slits 32, 33.

The device and the process are, in particular, suitable for assessingthe surface of an object, especially surfaces of flat objects such aswoods, tiles, textiles, glasses, plastic surfaces, foils, siliconwafers, cardboard and others. The device and process of the presentinvention is useful for identifying surface characteristics or defectssuch as shakes, fissures, cracks, holes, protrusions and indentations,and to evaluate such surfaces according to quality criteria. The deviceand process of the present invention are, in particular, suitable forinspection of woods, because they permit, for the first time, directevaluation of the scatter effect in the case of wood and provide aselective optical contrast enhancement, in the case of the most diversesurface defects, or enable a differentiation of woods with sawroughness. Equally, blue stain as well as dirt contamination or damageresulting from worms or cracks/shakes are made very well visible. Theusefulness of the invention consists in particular of the fact, thatwith it in real time various surface characteristics can be measured,such as:

a) the intensity distribution of the diffusely reflected laser light,and/or

b) the distribution of the intensity of the laser light scattered bylocal density variations (Tracheid effect), which is observed throughspatial filters in the scatter channel, and/or

c) the elevation profile (3D channel) of the surface, which is measuredby means of a triangulation process, and/or

d) double refraction characteristics, which are measured by detectionprocesses dependent on polarization, for example by means of an analyzerparallel and anti-parallel to the surface direction.

What is claimed is:
 1. A device for inspecting a surface of an object todetermine surface characteristics of the object, said device comprising:a scanning device, and an optical sensor unit comprising: a sendingmodule for emitting light to the scanning device, a receiving module forreceiving light incident from the scanning device, and an opticaldeflecting element that is operable to split light incident from thescanning device into a first beam path and a second beam path, saidfirst beam path being different than said second beam path, said firstbeam path being defined by a first spatially limited part of theincident light and said second beam path being defined by a secondspatially limited part of the incident light, and where the firstspatially limited part has a cross sectional area that is smaller than across sectional area of the second spatially limited part, wherein thescanning device includes: a light deflecting element, said lightdeflecting element having a time-dependent deflection angle, and aconcave mirror having a focal point and an axis of symmetry (7), whereinsaid light deflecting element is located in said focal point so thatlight emitted from the sensor unit can be guided into the sensor unitunder a constant angle relative to the symmetry axis of the concavemirror along a scanning line over the object and so that light diffuselyscattered from the object emanating from the scanning device can beguided into the sensor unit along a path identical to the emitted lightand wherein the concave mirror is disposed relative to the objectsurface and the receiving module such that the surface can betelecentrically projected into the receiving module and the lightdeflecting element acts as an aperture diaphragm, said receiving modulefurther comprising at least one light receiver with a special filter ora diaphragm designed and positioned in a manner that directly diffuselyscattered light is blanked out for said at least one light receiver, inorder to detect tracheid scattered light.
 2. The device according toclaim 1, wherein the deflecting element is a mirror with an aperture andis positioned such that a major portion of light traveling from theemitting module to the deflecting element passes through the aperture tothe scanning unit.
 3. The device according to claim 1, wherein thedeflecting element is a mirror and is positioned such that a majorportion of light traveling from the sending module to the deflectingelement is reflected to the scanning device by the mirror.
 4. The deviceaccording to claim 1, wherein the emitting module includes a pluralityof light sources that produce light having different wave lengths. 5.The device according to claim 4, wherein said plurality of light sourcesincludes at least a first light source and a second light source, saidfirst light source emits light having a wave length between about 620 nmand 770 nm, and said second light source emits light having a wavelength above 770 nm.
 6. The device according to claim 1, wherein thereceiving module includes an optical system, the optical system beingpositioned between said deflecting element and said at least one lightreceiver such that the at least one light receiver is disposed in thefocal plane of the optical system.
 7. The device according to claim 6,wherein the receiving module includes several light receivers and atleast one beam splitter, whereby by means of said at least one beamsplitter, light impinging on the receiving module can be split up amongthe light receivers.
 8. The device according to claim 7, wherein the atleast one beam splitter has a wave length dependent reflection totransmission ratio.
 9. The device according to claim 7, wherein afurther spatial filter or diaphragm is positioned ahead of a lightreceiver for blanking out light cones from a scatter effect.
 10. Thedevice according to claim 1, including two light receivers, each of saidtwo light receivers having a spatial filter or diaphragm arranged in amanner that directly diffusely scattered light is blanked out, saidspatial filters being positioned orthogonally relative to one another tomeasure directional dependence of tracheid scattered light.
 11. Thedevice according to claim 1, wherein the receiving module includes atleast one CCD camera or at least one position-sensitive receivingelement.
 12. The device according to claim 1, wherein the lightdeflecting element is a rotating polygonal mirror wheel.
 13. The deviceaccording to claim 1, wherein the concave mirror is a strip havingparallel cut edges out of a paraboloid.
 14. The device according toclaim 1, wherein, for obtaining a 3D profile by a triangulation process,at least one light beam emitted from the sensor unit can be guided on tothe object and light diffusely scattered from the object under aconstant angle (θ) relative to the incident beam can be guided back tothe sensor unit such that the incident beam and the guided back beam arecoincident in a plane parallel to the surface of the object.
 15. Thedevice according to claim 14, wherein, for obtaining a telecentric 3Dprofile by means of a triangulation process, the sensor unit includes aposition-sensitive receiving element capable of high speed, in order tomeasure the surface profile by deposition of the diffusely scatteredlight vertically to the object surface by means of telecentricprojection.
 16. The device according to claim 1, wherein the object ismovable relative to the sensor unit.
 17. A process for inspecting asurface of an object to determine surface characteristics thereof,wherein light emitted from a sensor unit is transmitted to a scanningdevice and the emitted light is guided to and deflected by a deflectingelement wherein the light deflected by the deflecting element is focusedon the object by a concave mirror having a focal point and a symmetryaxis, the light deflecting element being positioned at said focal point,said deflected light being guided under a constant angle relative to thesymmetry axis of the concave mirror along a scanning line over theobject, and light diffusely reflected from the object is generallyguided back to the sensor unit along the same path as the emitted light,wherein the object surface is telecentrically projected into the sensorunit by the concave mirror, whereby the light deflecting element acts asan aperture diaphragm and wherein additionally the intensity of tracheidscattered light guided back to the sensor unit is measured by areceiver, the directly diffusely scatted light being blanked out for thereceiver.
 18. The process according to claim 17, wherein the tracheidscattered light is measured by two receivers, each of said two receiverscomprising a spatial filter for blanking out the directly diffuselyscattered light point, the spatial filters being positioned orthogonallyrelative to one another to measure directional dependence of tracheidscattered light.
 19. The process according to claim 17, wherein forobtaining a 3D profile by a triangulation process, at least one lightbeam emitted by the sensor unit is guided on to the object and lightdirectly diffusely scattered from the object under a constant angle (θ)relative to the incident beam is guided back to the sensor unit suchthat the incident beam and the guided back beam are coincident in aplane parallel to the surface of the object.
 20. The process accordingto claim 17, wherein various surface characteristics are measured inreal time using parallel processors.
 21. The process according to claim17, further comprising separating light portions of differing wavelength impinging on the sensor unit from one another, andinstantaneously recording, from the light portions, with a highrepetition rate, various surface characteristics, said surfacecharacteristics including elevation profile in a 3D channel,reflectivity in a red light channel, and tracheid effect in a scatterchannel (15).
 22. The process according to claim 21, comprisingsplitting up light impinging on the sensor unit into first and secondlight portions, and guiding the first and second light portions to twolight receivers, said first light portion having the image of the lightcones of the scatter effect blanked out, the image of the directlydiffusely reflected light point on the object is evaluated and fromgrey-scale image is obtained, and in the case of the second lightportion, the directly diffusely reflected light point is blanked out byspecial spatial filters and only the image of the remaining light conesis evaluated by said receiver.
 23. The process according to claim 22,comprising the further steps of separating light portions of differingwave length impinging on the sensor unit from one another, whereby, witha first light portion in a triangulation process, 3D information ismeasured by means of a position sensitive receiving element and, with asecond light portion, the intensity distribution of the object as wellas the surface characteristics are measured with opto-electrical sensorelements.
 24. The process according to claim 17, wherein light impingingon the sensor unit makes density-dependent surface anomalies visibleand, by means of a four-quadrant process, which is a function of theposition (S_(x)+S_(y)) and the direction arctan (S_(x)+S_(y)) and, ifnecessary. in combination with the triangulation process in a 3Dchannel, which is a function of the elevation, is detected in a realtime process, the spatial resolution of which is limited only by afocusing ability of the light.
 25. The process according to claim 17,further comprising moving the object relative to the sensor unit.